This invention relates to a diaphragm device in which a diaphragm aperture is defined by portions of the inner peripheral edges of diaphragm blades which are arranged in partly overlapping relation to one another.
In those prior art diaphragm devices which provide diaphragm apertures defined by portions of the inner peripheral edges of a plurality of diaphragm blades arranged in partly overlapping relation to one another, the diaphragm aperture preferably defines a shape which is symmetric with respect to the center, i.e., the optical axis, so that the configuration of the aperture defined by the respective diaphragm blades are as close to a circle as possible.
However, the drawback with such an arrangement is that, when such a diaphragm is stopped down to its minimum aperture, the adjoining diaphragm blades interfere with each other, thus failing to provide a completely closed diaphragm. For this reason, a minimum diaphragm aperture necessarily results.
On the other hand, with a television camera, for instance, it has long been a demand that the light passing through an objective lens be blocked or shielded by means of a diaphragm device rather than a lens cap for the purpose of protecting the sensitive surface of a vidicon.
Still furthermore, with a color television camera provided with a color-encoding filter means for use in color separation, there results a beat between the striped color-signal carrier of such color encoding filter and the high frequency component of an image of a photographic object, thereby bringing about a false color. For this reason, an optical low pass filter having laminae which cause retardation of the light being transmitted therethrough is placed in an optical system of the television camera, thereby cutting off the high frequency components of an image formed through the objective lens, in an attempt to preclude the occurrence of a false color. (See U.S. Pat. Nos. 3,756,695 and 3,768,888 commonly owned herewith.)
However, such use of an optical low pass filter is only partly successful, i.e., when using the diaphragm device which provides a diaphragm aperture of a substantially circular shape about the axis of the optical path for stopping down the diaphragm beyond a given diaphragm aperture, so that the function of the optical low pass filter to cut off the high-frequency components of the image is sharply impaired.
Experimentation has shown that when using an objective lens having a focal length of 42 mm and an aperture ratio of 2.0 (aperture radius of 21 mm), and an optical low pass filter, in which the pitch of the grating is 1.9 mm, with the f-number of the diaphragm being set to over 11, then the function of the aforesaid optical low pass filter to cut off the high frequency component is radically impaired.
The relationship of the f-number to the diaphragm aperture for the aforesaid objective lens is as follows:
__________________________________________________________________________ f-number 2.8 4 5.6 8 11 16 22 __________________________________________________________________________ diaphragm aperture 14.9mm 10.5mm 7.4mm 5.2mm 3.7mm 2.6mm 1.8mm __________________________________________________________________________
This experimentation further proves that when the diaphragm aperture is of a value less than two times the pitch of the grating of the optical low pass filter, it may not be expected to maintain the intended function of the optical low pass filter. This is also true with the objective lenses other than those used in the aforementioned tests.
Also, in these tests, the f-number could not be set to a value greater than 22 due to the particular device as well as due to a ghost image produced by the diffraction phenomenon of an image beam, which results from the stopping down operation. Even when the f-number is set between 11 and 22, it is imperative that, for maintaining the intended function of the optical low pass filter, the pitch of the grating of the low pass filter be reduced to 0.9 mm. This in turn dictates the reduction in the width of a striped color signal carrier of a color-enoding filter. This however is physically impossible, so that the extent of stopping down the diagraphm for an objective lens is necessarily subject to certain limitations.
According to the present invention, there is provided a diaphragm device, in which, unlike the prior art diaphragm device providing a circular diaphragm aperture, the configuration of a resulting diaphragm aperture is substantially of an elongated circular shape having its length extending in a specific direction, with this specific direction being perpendicular to the direction of the grating, so that even if the diaphragm device is stopped down to any small diaphragm aperture, the aperture may include or cover two rows of the laminae of a grating at all times, so that the intended function of the optical low pass filter to cut off the high frequency components may be maintained, while permitting a stopping down of the diaphragm to a value greater than the f-number of 11.